The non-aqueous electrolyte secondary battery has a high energy density and a prolonged life as compared with the conventional batteries and thus has been used as a power supply for cellular phones, laptop computers, etc.
The non-aqueous electrolyte secondary battery comprises a carbon-based material as a negative active material, a lithium-transition metal composite oxide as a positive active material, and a non-aqueous electrolyte having a lithium salt dissolved therein as an electrolyte. In particular, the positive active material is an important element, which determines the performance such as discharge capacity, discharge voltage, cycle life performance, safety, etc. of non-aqueous electrolyte secondary battery.
A small-sized non-aqueous electrolyte secondary battery which is now commercially available comprises lithium cobalt oxide having a layered α-NaFeO2 type structure as a positive active material. However, cobalt, which is a starting material for lithium cobalt oxide, is a rare metal and its price is variable. Further, lithium cobalt oxide is much disadvantageous with respect to the safety of batteries during overcharging, etc. On the contrary, the lithium manganese oxide is a positive active material which is more favorable than other composite oxides in cost and safety of batteries because manganese, which is a starting material for lithium manganese oxide, occurs in abundance and exhibits an excellent thermal stability at high temperatures. Thus, the practical use of lithium manganese oxide is now under extensive study.
However, the lithium manganese oxide is disadvantageous in that its discharge capacity decreases as it is repeatedly charged and discharged. Thus, the lithium manganese composite oxide is not widely used in practice. This capacity drop is attributed to the volumetric change during the charge and discharge process, phase transition due to Jahn-Teller effect and elution of manganese ion at high temperatures. In particular, the elution of manganese ion at high temperatures causes remarkable capacity drop due to charge and discharge cycles at high temperatures and remarkable capacity drop due to storage at high temperatures.
In order to solve these problems, an approach is proposed involving the substitution of some of manganese elements by metals other than lithium and manganese. Many reports have been made on such an approach. The substitution of some of manganese elements by other elements allows the stabilization of the spinel framework structure, making it possible to reduce the elution of manganese and hence improve the cycle life performance of the battery.
However, in accordance with the foregoing approach involving the substitution of some of manganese elements by other elements, the cycle life performance at room temperature can be improved, but the cycle life performance at high temperature or the storage properties at high temperature should be further improved. Further, the effect exerted by the substitution of some of manganese elements by other elements depends greatly on the kind and amount of substituent metals. Thus, when the amount of substitution is insufficient, the resulting effect is insufficient even at room temperature. On the other hand, when the amount of substitution is raised, the discharge capacity per unit weight decreases. Thus, it is necessary that the cycle life performance of lithium manganese oxide be improved as much as possible, which does not depend on the substitution of metal ions.
It is therefore an object of the present invention to improve the cycle life performance, particularly cycle life performance and storage properties at high temperature, of lithium manganese oxide.